I. Field of the Invention
The present invention relates to communication systems. More particularly, the present invention relates to a method and apparatus for providing power control in a closed-loop communication system.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Although other techniques such as time division multiple access (TDMA), frequency division multiple access (FDMA), and AM modulation schemes such as amplitude companded single sideband (ACSSB) are known, CDMA has significant advantages over these other techniques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUTATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS," and assigned to the assignee of the present invention and incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM," assigned to the assignee of the present invention and incorporated by reference herein.
CDMA by its inherent nature of being a wideband signal offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user or mobile station through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501, entitled "METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM," and U.S. Pat. No. 5,109,390, entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM," both assigned to the assignee of the present invention and incorporated by reference herein.
If the transmission power of signals transmitted by the base station to a mobile station are too high, problems such as interfering with other mobile stations and expending unnecessary power are likely. Alternatively, if the transmission power of signals transmitted by the base station are too low, the mobile station can receive multiple erroneous transmitted frames. Terrestrial channel fading and other known factors affect the transmission power of signals transmitted by the base station, so that channel conditions change over time, especially as the mobile station moves. As a result, each base station must rapidly and accurately adjust the transmission power for the signals which it transmits to the mobile stations.
In a useful method for controlling the transmission power of signals transmitted by a base station, the mobile station measures the condition of such transmitted signals. The mobile station then transmits a signal corresponding to the measured power level, or compares the power level to a threshold value and transmits a signal or message to the base station when the power of a transmitted frame of data deviates from the threshold. In response to the received signal, the base station adjusts its transmission power of signals transmitted by the base station; if the power level is lower than desired, the base station increases the transmission power of its transmitted signals, otherwise, the transmission power is decreased. A method and apparatus for controlling transmission power is disclosed in U.S. Pat. No. 5,056,109, entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR TELEPHONE SYSTEM," assigned to the assignee of the present invention and incorporated by reference herein.
A delay necessarily exists under the above CDMA closed-loop communication system. The delay includes the time of the base station transmitting the frame with inadequate power, the mobile station receiving the degraded or erroneous frame, the mobile station recognizing the error (eg., the signal falls below the threshold or a frame is erased), the mobile station transmitting an appropriate error message to the base station, and the base station receiving the error message and appropriately adjusting its transmission power. The CDMA communication system, typically, transmits packets of data as discrete frames. As a result, the CDMA communications system cannot rapidly compensate for transmission power fluctuations. Additionally, the mobile station could request too great of an increase in the forward link transmission power before recognizing such an increase due to the delay.
Furthermore, the bandwidth of the channel over which the mobile station relays signals to the base station can be limited. For example, nearly all of the bandwidth for this channel is dedicated to transmitting voice traffic to the base station. Little or no bandwidth remains to transmit signals to the base station reflecting the measured power level of the signals transmitted by the base station. Therefore, such signals transmitted by the mobile station could only request that the base station increase or decrease the transmission power by a small amount. Consequently, if deep fades affect the transmitted signals, the base station may not be able to react quickly enough to increase its transmission power and calls can be undesirably terminated.
Information signals sent from the base station to a mobile station include voice information, error correction codes, identification codes and others. Increased bandwidth needed for power control information comes at the expense of the size of one or more of these other signals.
When transmitting from the mobile station to the base station, i.e., along the reverse link, the mobile station transmits signals in a series of frames. Each frame contains 16 power control groups. Each group contains an identical and predetermined number of encoded data symbols. A graphical showing of reverse link power control is shown in FIGS. 1A-1D, which shows power control group transmissions, respectively, at the full rate, half rate, quarter rate, and eighth rate.
When transmitting from the mobile station at the full rate, all 16 of the power control groups are sent. When transmitting at the half rate, however, only one-half of the 16 power control groups are sent (i.e., 8 power control groups). Accordingly, four power control groups are sent in one frame while transmitting in the quarter rate and only two power control groups are sent in one frame when transmitting at the eighth rate, as seen in FIG. 1D. All data is transmitted at a power level independent of rate.
The base station can only determine the sufficiency of the mobile station's transmitted power when it receives a control group. At the eighth rate, only two power control groups are sent every 20 milliseconds, meaning the base station can transmit power control commands twice every 20 milliseconds or 100 times per second. When changing transmission rates from the eighth rate to that needed to send high speed data, power control commands sent twice every 20 milliseconds are not sent often enough to accurately set the power of the mobile station transmitter.
Power control mechanisms on the forward link in this type of communication system use a different mechanism than that on the reverse link. Data is continuously being sent on the forward link regardless of data rate, as shown in FIGS. 1E-1H. FIG. 1E shows data being sent at the full rate, which includes 16 data packets in one frame. Data is repeated in the frame at rates lower than the full rate and the transmit power is scaled downward. FIG. 1F shows data being transferred at the half rate, which shows data packets 1-8 being sent twice in any given frame. FIGS. 1G and 1H show, respectively, packets transmitted at the quarter rate and the eighth rate. By repeating the data at the slower rates, while lowering the transmission power, the average power per frame per group remains substantially constant.
Since data is continuously being transmitted, feedback bandwidth of power control is not an issue, unlike the reverse link. When transmitting on the forward link, the mobile station does not know beforehand at what rate data is being sent. The energy of packets transmitted at the eighth rate may not be sufficient to enable the mobile station to make a correct power control decision. Additionally, the mobile station determines the rate sent by using internal algorithms. Errors in the rate determination will result in errors in determining the sufficiency of power.